1. Field of the Invention
The present invention relates to optometers and particularly to a Scheiner-principle vernier optometer apparatus, and method therefor, for measuring, e.g., the resting state of accomodation of a person in a darkened environment.
2. Description of the Prior Art
The need for precise lens accommodation to bring visual targets into sharp focus on the retina is far more urgent at night, when contrast is very low, than in bright daylight. But it is at precisely this time that many individuals become myopic and further reduce the quality of an already poor visual image. In many professions this phenomenon, sometimes called the "dark focus of accommodation", is of little consequence. But for some, for example pilots flying at night, it can mean the difference between life and death. A reliable screening apparatus capable of measuring the refractive state of individuals in the dark could, therefore, provide useful preventive information. Either the Laser-Badal optometer or the common vernier optometer could be used in this way, but both of them have problems as they are usually implemented.
Currently, much research in the area of "dark focus" has relied on the Laser-Badal optometer. This device is simple and produces accurate results. It is, however, quite difficult to use in practice, and some individuals are completely unable to produce data with this device. In addition, data taken from a Laser-Badal optometer is typically corrected for the chromaticity of the light source, usually a Helium-Neon laser with an output wavelength of 632.8 nm (nano meters), by adding 0.33 D (diopters) of myopia. This correction presumes to match the experimental result to one that would have resulted if the light source were at a wavelength of 585 nm (yellow light). A more direct approach is to use a 585 nm light source. However, laser light sources at this wavelength are not readily available.
An alternative optometer is the vernier optometer, which is based on the Scheiner principle and uses polaroids and a vernier task. A vernier optometer is simple to construct and to use. However, even a fairly sophisticated vernier optometer does not reliably produce accurate results.
The problems associated with prior art vernier optometers appear to occur for two reasons. First, prior art vernier optometers make no attempt to maintain the optical axis of the subject's eye in tight alignment with the optical axis of the vernier optometer. However, alignment of the subject's eye with the vernier image must be precisely controlled or readings will be discrepant. This phenomenon can be easily demonstrated by looking through an ordinary Scheiner-principle vernier optometer and intentionally moving the instrument or the eye in a direction perpendicular to the vernier image. The relationship of the vernier lines will also change. Second, the data taken from a vernier optometer are sensitive to the chromatic content of the light source used and/or any chromatic aberrations present in lenses or produced by small apertures. More particularly, a Scheiner-principle optometer requires a fair amount of light to get a usable image through the small Scheiner apertures, and a white light source is an easy way to get enough light through those small apertures. However, white light is, by definition, a collection of many different wavelengths, all of which are refracted to varying degrees by the human eye. These differences in refraction amount to about 0.8 diopters when comparing red and blue light sources in an optometer. So the problem in such Scheiner-principle vernier optometers is, if the light source is white, exactly which wavelength in the white light should be used as the criterion. Furthermore, in a vernier optometer based on the Scheiner-principle that uses a broad-band white light source, chromatic aberrations from the optometer lenses and pinhole apertures may be appreciable. These aberrations can become a serious source of error since the human lens is also known to exhibit appreciable chromatic aberrations.